NK cells play a pivotal role bridging the innate and adaptive immune systems and have been implicated in protection against pathogens and tumors. Contrary to prior expectations, recent studies have revealed that NK cells express a sophisticated and diverse system of inhibitory and activating receptors that regulate their behavior. Moreover, these receptors are rapidly evolving, presumably in response to pathogens as well as to newly arising polymorphisms in the host's MHC. The studies proposed in this application focus on the role of CD8alpha in human NK cell recognition of MHC class I and on the signaling and function of human KIR2DL4, a unique member of the human KIR gene family. Mouse NK cell do not express CD8 nor do mice possess KIR genes. The goal of specific aim 1 is to determine the functional significance of CD8alpha expression on human NK cells. The hypothesis to be tested is that human CD8a functions as a co-stimulatory or coinhibitory receptor for an activating or inhibitory KIR, respectively. We will also test the hypothesis that CD8alpha is more critical when low affinity KIR ligands are being recognized. The objective of specific aim 2 is to define the signaling pathway and functional properties of the human KIR2DL4 molecule. The hypothesis to be tested is that a novel signaling adapter protein is required for KIR2DL4 function. Furthermore, we propose that the downstream transcriptional targets of KIR2DL4 signaling will differ from other KIR2DS or KIR3DS activating receptors, which in turn will result in different biological outcomes. Therefore, our studies promise to provide new insights into immune recognition by human NK cells that cannot be readily addressed in mouse models. Our findings may have relevance to understanding the role of human NK cells and their receptors in infectious disease, cancer and autoimmunity.